Introduction to LNG Cooled Power Generation Technology

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The utilization of LNG’s cold energy relies primarily on the temperature and pressure differences between LNG and its surrounding environment; by converting high-pressure, low-temperature LNG into natural gas at normal pressure and temperature, the energy stored in LNG is recovered. LNG cold energy power generation is a way to efficiently utilize the cold energy of LNG. Common cold energy power generation processes include the direct expansion method, Rankine cycle method, combined method, Brayton cycle method, and gas turbine utilization method. PS: Anhui Danning Environmental Protection Technology Co., Ltd. is a turbine manufacturer. 1. In the storage tanks using the direct expansion method, the LNG is pressurized by a cryopump and then vaporized in an LNG evaporator, turning into a high-pressure, normal-temperature gas. The pressure energy generated during the high-pressure vaporization of the LNG is then used to directly drive a turbine expander, which in turn drives a generator to produce electricity. Features: Simple principle, low efficiency; the cold energy recovery rate is only 24%, and it utilizes only pressure exergy. https://pic1.zhimg.com/80/v2-29ac8d92754590f2dde6c2b634bb9ab4_1440w.webp Figure 1: Direct expansion method. 2. Rankine cycle method: The Rankine cycle method is also known as the intermediate medium Rankine cycle method or the secondary medium method. The low-pressure refrigerant resulting from the expansion of LNG in a turbine (such as propane) exchanges heat in a condenser, where the refrigerant condenses into a liquid state ； The low-pressure refrigerant liquid has its pressure increased by a pump, and then it is heated to turn into high-pressure steam ； The high-pressure refrigerant vapor is expanded by a turbine into low-pressure vapor, which generates power to drive a generator to produce electricity. Cold energy recovery efficiency for a single working fluid: 18% ； The cold energy recovery efficiency of the mixed working fluid is 36%. https://pic1.zhimg.com/80/v2-54ea819ccbf2cbfdd13cc7ca946e527c_1440w.webp Figure 2: Rankine cycle method. 3. Combined method: The combined method integrates the direct expansion method with the Rankine cycle method. LNG transfers its cold energy to the refrigerant through heat exchanger 2; after passing through heat exchanger 3, it becomes a high-pressure, room-temperature gas. It then expands within turbine 2 to generate power that drives motor 2 for electricity production. Finally, after passing through heat exchanger 4, it turns into a gas at a certain pressure and room temperature before being exported. The refrigerant (such as propane) is liquefied, compressed by pump 1 and passed through a regenerator to become a high-pressure gas; it is then compressed by pump 2 and passed through heat exchanger 1 to become a high-pressure, normal-temperature gas. Finally, it drives motor 1 via turbine 1 to generate electricity, and the resulting refrigerant is recycled again. https://pic3.zhimg.com/80/v2-b743e0ec0ae5d27a776cdc967388ff32_1440w.webp Figure 3: Combined approach 4. Hybrid-media power generation. Since the temperature of LNG changes throughout the process, using hybrid media allows it to harness the cooling energy over a wider temperature range compared to using a single medium, thereby enabling more efficient utilization of the LNG’s cooling energy. However, due to the inherent instability of mixed media, this method encounters many difficulties in practical applications. 5. The Brayton cycle method is also known as the gas power cycle method. In the process flow diagram below, on the left is the Brayton cycle using N2 as a working medium under low-temperature conditions, while on the right is direct LNG expansion power generation. The inlet gas to the compressor is cooled using the cold energy of LNG, thereby reducing its temperature; as a result, the compressor requires less power to achieve the same pressure increase. The high-pressure N2 is then heated by a heater and fed into a gas turbine to do work, generating electrical power in the process, which significantly improves the thermal efficiency of the system. https://pic2.zhimg.com/80/v2-3777af7533312041c9e5b1a8aa3064d5_1440w.webp Figure 4: Brayton cycle method. 6. Gas turbine utilization method: Different refrigerants are used to harness the cooling energy released during the vaporization of LNG, either directly or indirectly, in order to reduce the temperature of the air entering the gas turbine or to cool the exhaust gas from the steam turbine. By using the cooling energy of LNG to cool the gas entering the compressor, the thermal efficiency of the system is significantly improved. https://pic3.zhimg.com/80/v2-c0b6f6fb7448ef362905f0e6006e9fa2_1440w.webp Figure 5: Utilization methods of gas turbines. 7. Comparison of several power generation processes. The Brayton cycle has the highest power generation efficiency, which can reach 55%; however, it requires a certain temperature for the cooler ； The Rankine cycle method (using a single working fluid) has the lowest power generation efficiency, at only 18% ； The power generation efficiency of the Rankine cycle method (mixed working fluid) is 36% ； Although the direct expansion method has a low efficiency in utilizing cold energy, at only 24%, its principle is simple. The Rankine cycle method, combined method, and gas turbine method have lower efficiencies than the Brayton cycle method, but they offer greater versatility, fewer constraints, and are more technologically mature, making them worthy of promotion. Overall, the efficiency of LNG cold energy utilization both domestically and internationally is currently low, and there are many shortcomings in the power generation methods, which require further improvement. Table 1 Comparison of Characteristics of Six Power Generation Processes | Advantages | Disadvantages | Applicability |
|-----------------------------------|------------|---------------|----------------|
| Direct expansion method | Simple principle | Low efficiency in utilizing cold energy, low power generation | Used for recovering some cold energy; small LNG regasification plants, low-pressure natural gas |
| Rankine cycle method | High efficiency; efficiency can reach 36% | Cold energy from natural gas above the condensation temperature is not utilized; complex process | |
| Combined method | Efficiency up to 50%; low overall cost; environmentally friendly | The working medium itself is unstable, making practical application difficult | |
| Mixed-media power generation | Can utilize cold energy over a wider temperature range of low-temperature natural gas; allows for hierarchical utilization of LNG’s cold energy | | Large LNG regasification plants |
| Brayton cycle method | Efficiency above 50% | Complex process | Cooling temperatures above 0°C |
| Gas turbine utilization method | Significantly improves thermal efficiency | | Large LNG regasification plants |

A comparison chart of the efficiencies of various power generation processes is shown below: https://pic2.zhimg.com/80/v2-88b4160aaed31c3fe51d32370ee0006d_1440w.webp
Figure 6: Comparison of efficiencies of various power generation processes

III. LNG Cold Energy Power Generation Equipment
Taking the relatively mature Rankine cycle method as an example, LNG cold energy power generation systems mainly consist of low-temperature turbine expanders, regasifiers, and low-temperature pumps. 1. Low-temperature turbine expander: The turbine expander is a key device for generating electricity using LNG’s cold energy, and it serves as the core element that ensures the stable operation of the entire system. Its main principle is to utilize gas under certain pressure to undergo adiabatic expansion within the volute of a turbine expander, thereby doing work and consuming the internal energy of the gas itself. Turbine expanders can be divided into axial flow and radial flow types based on their structure. However, with the exception of axial-flow expanders that are used in applications involving high flow rates, high power levels, and high temperatures, the vast majority of turbine expanders are of the centrifugal radial-flow type, and they can be designed as single-stage or multi-stage depending on the requirements. https://pic4.zhimg.com/80/v2-651035727ef6d937bf48d8cb04623d9f_1440w.webp Figure 7: Low-temperature turbine expander. https://pic4.zhimg.com/80/v2-450bb3cac334148a8c042d6bd247afd7_1440w.webp Figure 7: Low-temperature turbine expander. The main imported brands of turbine expanders currently available include GE Siemens, the French company CRYOSTAR, the American company ADC, and the Swedish company Atlas. Among domestic brands, there is “Xin” produced by Anhui Danning Environmental Protection Technology Co., Ltd., as well as other brands such as Lianyou Machinery, Chuan Kong, Hang Yang, and Ruite. 2. LNG cryopump: The LNG cryopump is also a key device in LNG cryogenic power generation, and its performance parameters as well as operational stability are crucial for the safe and reliable operation of the entire system. Currently, the main imported brands include France’s CRYOSTAR, Switzerland’s Cryomec, the USA’s ACD, Japan’s Ebara, and Japan’s Nikkiso. In China, manufacturers such as Dalian Deep Blue and Hangyang also produce such products. Taking the propane circulation pump as an example, it is used in cryogenic power generation systems as a pump for absorbing and transporting the propane circulating fluid; its head is set at a low level, mainly to overcome the pressure drop in the circulation system. The propane circulation pump is a vertical submersible pump, designed to operate continuously at full load. The structure and appearance of the propane circulation pump are shown in the figure below. https://pic2.zhimg.com/80/v2-afb08de814da676375e1b528e6749e8d_1440w.webp Image 8: LNG cryopump https://pic2.zhimg.com/80/v2-6611a9cc789d553c86ca94fbfa75ef05_1440w.webp Image 8: LNG cryopump. 3. Vaporizers: The LNG vaporizers currently in use come in the following types: open-type vaporizers (ORV), submerged combustion vaporizers (SCV), intermediate medium vaporizers (IFV*propane), and intermediate medium shell-and-tube vaporizers (IFV–forced circulation). In gasifiers of the aforementioned types, open-type gasifiers and submerged combustion gasifiers are widely used. Intermediate medium gasifiers are also commonly employed when the quality of seawater does not meet the requirements of open-type gasifiers, or when there is waste heat from power plants available near the reception station and cooling energy is needed by other process facilities. (1) Open-type vaporizer (ORV): An open-type vaporizer (ORV) is a vaporizer that uses water as a heat source, and it is used in large-scale vaporization units for basic load applications. The current maximum processing capacity is 250 t/h. The vaporizer can operate within a load range of 0% to 100%. https://pic1.zhimg.com/80/v2-a5fa37ec14e7cd8ff26dac522ac50acc_1440w.webp Figure 9: Open-type vaporizer (ORV) (2). Submersion combustion vaporizer (SCV): The submersion combustion vaporizer (SCV) uses its own natural gas as a heat source; therefore, it has higher operating costs. Typically, SCVs consume 1% to 2% of natural gas as fuel. Due to its simple structure, it has an advantage in terms of equipment investment costs. https://pic4.zhimg.com/80/v2-ac7f0cd9551f4b6f198b1e5364d1e527_1440w.webp Figure 10 Submersion combustion vaporizer (SCV) (3) Shell-and-tube vaporizer/intermediate fluid vaporizer (STV/IFV). STV/IFV (IFV is also a type of shell-and-tube vaporizer), has the advantage of allowing the use of various heat sources, such as water, air, and industrial waste gases. In particular, IFVs can use media such as propane, butane, or Freon as intermediate heat transfer fluids, which helps to mitigate the effects of icing. Currently, this type of vaporizer has been applied in LNG vaporization systems, with a maximum natural gas processing capacity of 150 t/h. However, if the water temperature and ambient temperature at the receiving station are very low in winter, and there are no heat sources nearby, then the STV/IFV cannot serve as a backup vaporizer for the receiving station. The figure below shows the intermediate-fluid gasifier of the IFV. https://pic2.zhimg.com/80/v2-8e542ed90228020ec5812b94f07cda69_1440w.webp Figure 11: Shell-and-tube vaporizer/intermediate fluid vaporizer (STV/IFV) https://pic1.zhimg.com/80/v2-b9257ca60ce030bae2c124091bec0e5c_1440w.webp Figure 11: Shell-and-tube vaporizer/intermediate fluid vaporizer (STV/IFV) Differences and applications of various vaporizers: Table 2 Comparison of characteristics of different vaporizers. Vaporizer type: ORV, SCV, IFV, STV. Intermediate medium: Water, propane, propane or alcohol-based liquids. Heating medium: Water, fuel gas, water, air/water/fuel gas. Process complexity: Simple, relatively complex, complex. Equipment complexity: Simple, relatively complex, combined and complex. Operation control complexity: Simple, simple, simple, relatively complex. Footprint: Smallest, least, less, larger. Applications: Widely used; mostly for peak shaving; in Japan it is used for energy recovery – 20 units; also used for energy recovery – 5 units. The main manufacturers of vaporizers for LNG receiving stations currently include Kobe Steel (Kobelco) from Japan, Tokyo Gas from Japan, and Sumitomo from Japan ； The ORV vaporizer, jointly and independently developed by China National Offshore Oil Gas & Power Group Co., Ltd. and Jiangsu Zhong Sheng High-Tech Industry Co., Ltd., has passed the scientific and technological achievement appraisal organized by the China Petroleum and Chemical Industry Federation. This achievement is highly innovative, fills a domestic gap, and generally meets international advanced standards. This achievement has been successfully applied to CNOOC’s Hainan LNG receiving station and Sinopec’s Beihai LNG receiving station, enabling industrial use in China. (Source: Li Xuejun, Deputy Chief Engineer of China Energy Engineering Group Hunan Institute)